This invention relates to a process for reducing the concentration of undesired sulphur containing impurities and, preferably, of undesired halogenated organic compounds in (hydro)halocarbon compositions.
(Hydro)halocarbons typically have a slight ethereal odour. Contamination by sulphur containing impurities can cause a momentary or even a lingering malodour.
Malodour can be caused both by inorganic and organic sulphur containing compounds, such as hydrogen sulphide, carbon disulphide, carbonyl sulphide, sulphur dioxide, sulphur trioxide, sulphuric acid, dimethyldisulphide, ethanethiol and diethyldisulphide.
Sulphur containing impurities may be introduced to (hydro)halocarbons during manufacture, for example from contaminants in hydrofluoric acid. Hydrofluoric acid may contain sulphur containing contaminants such as sulphur dioxide, sulphur trioxide, hydrogen sulphide and sulphuric acid, which may react with (hydro)halocarbons and/or their precursors during production to form sulphur containing impurities such as dimethyldisulphide, ethanethiol and diethyldisulphide. Some of these impurities survive known separation processes and reside in the bulk material.
Malodour can be unpleasant in any use of (hydro)halocarbons, particularly in dispensive uses and most particularly in pharmaceutical products where the (hydro)halocarbon is taken into the body.
It is, therefore, desirable to remove sulphur containing impurities from (hydro)halocarbons, such as halogenated alkanes, alkenes and ethers. Such removal is particularly important for pharmaceutical grade products, such as those used as propellants in pressurised metered dose inhalers.
Purification techniques that are well known in the art, such as distillation, are typically unsuitable for removing sulphur containing impurities from (hydro)halocarbons. Distillation does not achieve the very high purities required to provide (hydro)halocarbons without malodour.
Known processes, therefore, typically do not remove all of the malodour causing sulphur containing impurities.
Techniques used in the art to remove halogenated organic impurities from halogenated alkanes include the use of molecular sieves. Examples of the use of molecular sieves to remove such impurities are described in U.S. Pat. No. 6,274,782, U.S. Pat. No. 4,906,796 and U.S. Pat. No. 5,288,930.
U.S. Pat. No. 6,274,782 describes the separation of 1,1-difluoroethane (R-152a) from hexafluoroethane (R-116) using a combination of molecular sieves 5A, 10X and 13X.
U.S. Pat. No. 4,906,796 describes the purification of 1,1,1,2-tetrafluoroethane (R-134a) using carbon molecular sieves and zeolites. The process described removes chlorodifluoroethene (R-1122) from R-134a.
U.S. Pat. No. 5,288,930 describes the removal of chlorodifluoroethene (R-1122) from 1,1,1,2-tetrafluoroethane (R-134a) using a zeolite having a pore size of from 3.5 to 4.8 Å with potassium as the counter ion.
The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.
The molecular sieves previously used to remove halogenated organic impurities from halogenated alkanes typically do not also remove sulphur containing impurities to the extent necessary to remove malodour.
There is, therefore, a need for a process that effectively and efficiently reduces the concentration of sulphur containing impurities or removed these impurities from (hydro)halocarbons.
The present invention provides a new process for reducing the concentration of at least one undesired sulphur containing impurity and, preferably, of at least one undesired halogenated hydrocarbon from a (hydro)halocarbon composition.